This invention relates to an electrostrictive element or transducer of a multilayer structure which is generally called a stacked chip or ceramic capacitor structure in the art. An electrostrtictive element according to this invention is specifically useful in, among others, a printer head of an impact printer or a relay.
As described in a report contributed by Roderic Beresford to "Electronics," Nov. 3, 1981, pages 39 and 40, under the title of "Piezoelectric bender actuates tiny relays and dot-matrix printers," an electrostrictive element is useful as a printer head, a relay, or the like. An electrostrictive element is actuated by a d.c. voltage repeatedly supplied as voltage pulses when used in such fields of application. The element must withstand repeated application of such voltage pulses. In other words, the element must have a long life when a great number of voltage pulses are applied. Furthermore, it is desirable that the element give rise to a large displacement.
An electrostrictive element of a simplest type comprises an electrostrictive piece of a material capable of exhibiting a strong electrostrictive effect having a pair of electrodes on respective principal surfaces. When a d.c. voltage is supplied between the electrodes to produce an electic field, the electrostrictive material elongates and contracts in both the direction of the electric field and transversely. Such deformations or strains in the direction of the electric field and in the transverse direction are called the longitudinal and the transverse electrostrictive effects, respectively. It is known that the longitudinal electrostrictive effect ordinarily gives rise to two to three times as great a deformation as the transverse electrostrictive effect. The longitudinal electrostrictive effect therefore provides a higher efficiency of conversion from electric energy to mechanical energy. The deformation in the longitudinal or the transverse direction depends on the field intensity of the electric field produced in the electrostrictive piece.
When the transverse electrostrictive effect is used, it is possible with a certain applied voltage to achieve a displacement in the transverse direction which is proportional to the transverse dimension of the electrostrictive piece. When the longitudinal electrostrictive effect is used in order to take advantage of its higher efficiency of energy conversion, the displacement in the longitudinal direction does not grow for a given voltage with an increase in the dimension which the piece has in the longitudinal direction. This is because the field intensity becomes weak as the longitudinal dimension increases. It is therefore necessary, for attaining a great displacement with the longitudinal electrostrictive effect, to raise the applied voltage, so as to strengthen the field intensity. A power source for a high voltage is, however, bulky and expensive. Furthermore, a high voltage is objectionable in view of the danger inevitable during operation of the electrostrictive element and also in view of the withstanding voltage of IC's which are used in a driving circuit for the element. The element is therefore given the multilayer structure when the longitudinal electrostrictive effect is used.
As will later be described in detail with reference to two of nearly thirty figures of the accompanying drawings, a multilayer electrostrictive element comprises a plurality of electrostrictive sections defined in a stack or lamination by a plurality of internal electrodes which are perpendicular to an axis of the stack. The stack has a peripheral surface which has a predetermined outline perpendicularly to the axis. For use in a printer head or a relay, it is preferred that the predetermined outline be four sides of a rectangle. A first external electrode is connected to alternate ones of the internal electrodes and is placed externally of the peripheral surface. A second external electrode is connected to others of the internal electrodes and is positioned externally of the peripheral surface. At least one of the first and the second external electrodes may be extended in contact with the peripheral surface. Even in this event, it is possible to understand that the external electrode or electrodes are situated externally of the peripheral surface.
With a multilayer electrostrictive element, a great displacement is achieved at a low voltage even by the use of the longitudinal electrostrictive effect. It is to be noted here that each internal electrode has an internal electrode outline which is positioned only partly on the peripheral surface for connection to the first or the second external electrode. In other words, each internal electrode has an internal electrode area which is considerably narrower than a cross-sectional area of the stack. As will later be discussed with reference to several figures of the accompanying drawing, it has now been confirmed that this results in a short life which the electrostrictive element has when repeatedly supplied with voltage pulses. Moreover, this restricts the displacement, as compared with the displacement which is theoretically attainable.